1. Field of the Invention
The subject invention relates to a process for the preparation of polyurethane polyurea polymers by the reaction injection molding (RIM) technique without the necessity of utilizing external mold release agents. More particularly, the subject invention relates to the use, as chain extender in the RIM process, of a mixture the major proportion of which consists of a composition containing both sterically hindered primary aromatic diamines and reactive primary aromatic diamines.
2. Description of Related Art
Federal Republic of Germany published application No. 11 96 864 (U.S. Pat. No. 3,099,516) discloses the preparation of cross-linked plastics using isocyanate addition polymerization wherein hydroxyl group-containing compounds and polyisocyanates are expanded in molds in the presence of blowing agents and catalysts.
By correctly selecting the hydroxyl group-containing polyesters, polyethers, polyester amides, etc. and the organic polyisocyanates and in addition using chain extenders such as glycols or diamines, this method can be used to produce both elastic and rigid polyurethanes as well as modifications between these extremes.
Federal Republic of Germany Pat. No. 831 604 (U.S. Pat. No. 2,778,810) discloses the preparation of polyurethane elastomers from hydroxyl group-containing compounds and polyisocyanates by first preparing isocyanate group-containing prepolymers, which are then reacted in a second step with chain extenders to form higher molecular weight elastomers.
The use of diamines as chain extenders generally prohibited the use of a one-shot process for the preparation of polyurethane polyurea polymers. As disclosed in Federal Republic of Germany published application No. 11 49 523 (U.S. Pat. No. 3,105,062), crystalline, aromatic primary diamines are incorporated in the liquid, isocyanate group-containing prepolymers in less than the stoichiometric amount relative to the isocyanate groups, at a temperature less than the melting point of the diamines. The resulting mixtures are then cured by the application of heat. In Federal Republic of Germany Pat. No. 12 40 654 (U.S. Pat. No. 3,428,610), the isocyanate group-containing prepolymers are reacted at room temperature or moderately elevated temperatures with liquid or dissolved aromatic diamines which contain at least 1 linear alkyl substituent in the ortho position relative to the first amino group and two linear alkyl substituents having from 1 to 3 carbon atoms on the second amino group.
A process for the preparation of optionally cellular, elastic molded parts having a closed skin from polyurethane polyurea elastomers using the principle of reaction injection molding (RIM) is described in Federal Republic of Germany published application No. 26 22 951 (U.S. Pat. No. 4,218,543). The systems cited therein consist primarily of organic polyisocyanates, polyols, and reactive di- or polyamine chain extenders substituted by alkyl groups in the ortho position relative to the amino group. Strong catalysts for the reaction between the hydroxyl and isocyanate groups are present. In this process it is essential that the aromatic di- or polyamines are miscible in all proportions with polyols having molecular weights from 1200 to 1800. Furthermore, it is essential that the alkyl substituents must have from 1 to 3 carbon atoms; at least two of the alkyl substituents have from 2 to 3 carbon atoms; and each of the ortho positions relative to the amino groups must be substituted. Such systems may possess cream times of less than one second. With these low cream times, the transition from liquid to solid phase takes place almost instantaneously, which causes the liquid reaction mixture to solidify uniformly on the walls of the molds.
It is also known that the reactivity of aromatic amino groups can be sharply reduced relative to isocyanates through the use of electrophilic substituents. Examples of such aromatic diamines, as cited in Federal Republic of Germany Pat. No. 12 16 538 (British Pat. No. 981,935), are 3,3'-dichloro-4,4'-diaminodiphenylmethane, 3,3'-dinitro-4,4'-diaminodiphenylmethane and 3,3'-dichloro-4,4'-diaminodiphenyl. The use of such compounds is subject to expensive handling restrictions based on health and safety regulations. The use of the highly electronegative substituents in these compounds, however, reduces the reactivity of the aromatic amino groups to such a large extent that curing of the reaction injection molded parts can require up to 15 minutes, thereby making the use of this process uneconomical.
Polyurethane polyurea formulations having reduced reactivity and therefore improved flowability compared to systems prepared in accordance with Federal Republic of Germany published application No. 26 22 951 are disclosed in the specifications of European published application No. 26 915. According to this process, 3,3',5,5'-tetraalkyl substituted 4,4'-diaminodiphenylmethanes in which the alkyl radicals are the same or different and represent a methyl, ethyl, isopropyl, sec- or tert-butyl radical, and wherein at least one of the substituents must be an isopropyl or sec-butyl radical are used as the aromatic diamines. The tetraalkyl-substituted diaminodiphenylmethanes mix quite well with the polyols in the required amounts at room temperature and exhibit only slight or even no tendency to crystallize, so that the formulations are easily handled under the standard conditions for conventional RIM systems. However, it has also been found that the tetraalkyl-substituted 4,4'-diaminodiphenylmethanes may not be reactive enough for some applications.
Somewhat more reactive polyurethane polyurea formulations are described in European published application No. 69 286. Trialkyl-substituted meta-phenylenediamines are used as the aromatic diamines, wherein two of the alkyl substituents may be the same or different and are linear or branched alkyl radicals having from 1 to 4 carbon atoms while the third alkyl radical has from 4 to 12 carbon atoms or is a 5- or 6-membered cycloalkyl radical. Even with a relatively high diamine content, the formulations exhibit sufficient flowability and produce molded parts which have high dimensional stability at elevated temperature, and which do not exhibit a progressive drop in their shear modulus curves between 100.degree. C. and 200.degree. C.
The use of polyoxyalkylene polyamines having molecular weights from 1100 to 16,000 for the preparation of elastic polyurethane or polyurethane polyurea elastomers is disclosed in European Published application No. 81 701 and Federal Republic of Germany published applications Nos. 32 15 909 and 32 15 907.
In addition to such other important factors such as cream time and maintenance of acceptable physical properties, mold release factors are highly important. The cited processes have the disadvantage that external and/or internal release agents must be used as demolding aids when the parts are produced in standard production, since the molded parts otherwise stick to the mold and can be damaged during demolding. The use of internal release agents produces only a slight improvement in self-releasing characteristics. Even when polyoxyalkylene polyamines are used as disclosed in European published application No. 81 701, internal release agents are still required before the use of external release agents can be eliminated. If these release agents are not used, then damage to the molded part can be expected after from 5 to 10 demoldings. However, the use of external release agents makes it much more difficult or even impossible to paint the molded parts.